Detailed analysis of the variable star population in the globular cluster NGC 7006

Abstract

ABSTRACT A 5-yr CCD photometric VI time series of NGC 7006 is employed to perform a detailed analysis of the known population of variable stars. In the process, we have corrected inconsistent classifications, sky coordinates and found 10 new cluster member variables. An independent reddening estimate with a value E(B−V) = 0.08 ± 0.05 is made. Using Fourier decompositions of RR Lyrae light curves and well-established calibrations, the cluster mean metallicity and distance [Fe/H]ZW = −1.53 ± 0.15 and 41.2 ± 1.4 kpc are estimated based on an extended sample of cluster member RRab stars. Using the Gaia DR3 data, we performed an extensive membership analysis that leads to a clean colour–magnitude diagram, and hence to the identification of variables that are likely field stars, and to considerations on the variable distribution in the horizontal branch (HB). A double-mode RR Lyrae and three CW stars are discussed. The origin of CW stars from precursors in the blue tail of the HB with very thin (∼0.06 ± 0.01 M⊙) envelopes is argued. Our models indicate that the main-sequence predecessor of RR Lyrae stars had a mass of 0.82–0.85 M⊙ and lost about 25–35 per cent of its mass during the red giant branch events before settling in the HB some 12–13.5 Gyr later.